In the radio communication system in which a fixed number of frequencies are assigned and used repeatedly to service areas, which are arranged to cover continuously the surface, to carry out the multiple access between one base station provided in each service area and a plurality of subscriber stations by virtue of the TDMA method, since the service areas which employ the same frequency as that of user's own station are arranged at a distance, interference of radio waves which occurs between the service areas utilizing the same frequency must be prevented.
As an example of a means for preventing the interference between the service areas, the technology for tilting downward the antenna directivity of the base station is described in “Foundation of Mobile Communication” Chapter 10, Paragraph 10.2.5, supervised by Yoshihisa Okumura and Masaaki Shinji and published by the Institute of Electronics, Information and Communication Engineers, 1986.
FIG. 1 is a diagram showing a method which avoids the interference by splitting the service area into six sectors and then switching the antenna whose horizontal-plane directivity is narrowed (referred to as a “sector antenna” hereinafter) to respective sectors.
More particularly, a base station 1 can avoid the situation that an interference wave 51 incoming from other service area 921 which is located on the opposite side to the subscriber station is received, by using a third sector antenna 113 to communicate with a subscriber station 21 and using a fourth sector antenna 114 to communicate with a subscriber station 22.
Also, as for the down multiple address channel (Bch) used to transmit the message from the base station 1 to the subscriber stations 21, 22, the transmitting sector antenna is controlled to be switched sequentially, such as an antenna 111 in a TDMA frame 31, an antenna 112 in a TDMA frame 32, etc. Therefore, each of the subscriber stations 21, 22 can receive surely the multiple address channel once in the period of TDMA super frame 3. Similarly, as for the up access request channel (Rch) used to transmit the message from the subscriber stations 21, 22 to the base station 1, the subscriber stations 21, 22 can receive the up access request channel once in the period of TDMA super frame 3 without fail.
In the meanwhile, as an example of the method which detects and avoids positively the interference, procedures are decided in the second-generation cordless telephone system (PHS) such that a base station (CS) and a mobile station (PS) measure the FER (frame error rate) of the received signal respectively, then recognizes that interference occurs in the concerned communication channel if the FER exceeds a predetermined value, and then carries out an interference avoidance operation in the order of switching the channel to other channel on the same carrier, switching the channel to a channel on other carrier, switching the channel to a channel of other base station, automatic reconnection, temporary stop of transmission, and release of the radio channel, in response to the level of FER. These channel switching operations are carried out when the base station issues the instruction to the mobile station via a control channel. Also, if interference is detected by the mobile station, the channel switching is carried out when the mobile station issues the channel switching request to the base station.
FIG. 2 is a diagram showing an example of a TDMA frame format in such radio communication system. A TDMA frame 30 consists of four kinds of TDM channels 40 each having the different purpose, e.g., B channel 41, R channel 42, Uu channels 431 to 43n, and Ud channels 441 to 43n. 
The B channel 41 is the multiple address channel directed from the base station to the subscriber station, and used to inform the subscriber station of a frame timing synchronization and control information common to the subscriber stations. The R channel 42 is a channel used to issue the connection request from the subscriber station to the base station, and has normally a method to which respective subscriber stations can access at random. Accordingly, the collision of communication bursts occurs due to random access in the R channel. However, in order to reduce the collision occurring probability, as shown in FIG. 3, there exists the system in which a plurality of R channels (R1) 421 to (R4) 424 are arranged in one TDMA frame 30. Uu's are up user channels which are set one by one between the subscriber stations and the base station, and Ud's are similar down user channels. Each user channel consists of a preamble (PR) 61, an individual control information time slot (C) 62, and time slots (D) 631 to 63n having the number which corresponds to an amount of information.
In addition, in the method which measures an error rate of the received signal to detect the interference and then performs re-transmission of the interfered packet, change of the communication channel, etc., if the random access type channel such as the R channel is present, it is difficult to distinguish the interference from the collision because the collision of the transmitting bursts is assumed as the premise in such channel. In order to overcome this problem, based on the event the interference wave is not synchronized with the transmitting burst, an approach may be utilized which decides the receiving burst as the collision if such receiving burst lacks in all time slots and decides the receiving burst as the interference if such receiving burst lacks partially.
However, in the radio communication system which performs the multiple access between one base station and a plurality of subscriber stations by virtue of the TDMA method, if the surface service is intended to achieve, the avoidance of the interference only by using the antenna directivity is limited because of the topographical complexity of the service areas, etc. In addition, the system typified by the PHS, in which both the base station and the subscriber stations make arrangements to avoid the interference, is effective as the system which avoids the interference in the user channels, but such system cannot effectively function in the channel, which is used commonly by a plurality of subscriber stations, such as the multiple access channel, the random access type channel, etc.
Further, in the system typified by the subscriber radio access system, which has a large transmission capacitance per channel and in which the burst length of the user channel is varied in response to the service, there are the cases where the burst length of the interference wave is longer several times than the random access channel. Therefore, this system contains such a problem that, if all time slots of the random access channel are left out, it is impossible to distinguish the interference from the collision.
Moreover, in the case of the communication system in which the base station and the subscriber stations are fixedly operated, since both the interfering station and the interfered station are not moved, the influence of the interference lasts for a long time once they are affected by the interference.
As a result, the avoidance of the interference onto the multiple access channel and the random access channel is difficult, and the particular subscriber stations get into the situation that they cannot be connected for a long time once the interference occurs.
The present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for detecting precisely occurrence of the interference in the TDMA radio communication and provide a method for avoiding effectively the interference.